[API] Change enums to be const enum -- unless necessary to have the full enum name at runtime

[MSNev]

• This only affects the JavaScript OpenTelemetry library
• This may affect other libraries, but I would like to get opinions here first

To better support / reduce code size can we change all of the enum values to be const enum, unless there is an absolute need for reverse lookups at runtime from the enum "name" to it's value. And as part of the API no actual object results (like interfaces) in the final code.

In most cases this should be a non-breaking change, except for any reverse lookups. This should be done before GA to avoid anyone taking dependencies on the full name and creating their own reverse lookups based on the default const object created by TypeScript.

eg
(Would need to add explicit values to avoid accidental breaks from insertions in the future)

export const enum SamplingDecision {
NOT_RECORD,
RECORD,
RECORD_AND_SAMPLED
};

Usage becomes

if (sampling === /* SamplingDecision.RECORD */ 1) {
}

[MSNev]

This looks like it's a potential duplicate of #538, but it should be done before GA